Black hole caught shredding star

Posted by Kate Taylor

Astronomers have had their best look yet at a supermassive black hole in the process of shredding a star.

Using NASA's Galaxy Evolution Explorer and the Pan-STARRS telescope in Hawaii, astronomers have identified it as a star rich in helium gas, in a galaxy 2.7 billion light-years away.

"When the star is ripped apart by the gravitational forces of the black hole, some part of the star's remains falls into the black hole while the rest is ejected at high speeds," says Suvi Gezari of the Johns Hopkins University.

"We are seeing the glow from the stellar gas falling into the black hole over time. We're also witnessing the spectral signature of the ejected gas, which we find to be mostly helium. It is like we are gathering evidence from a crime scene. Because there is very little hydrogen and mostly helium in the gas, we detect from the carnage that the slaughtered star had to have been the helium-rich core of a stripped star."

Interestingly, it looks as it it's not the first time this particular star has encountered the black hole. The team believes the hydrogen-filled envelope surrounding its core was lifted off a long time ago by the same black hole.

Nearing the end of its life, it had probably ballooned in size, becoming a red giant in a highly elliptical orbit. On one of its close approaches, they say, the star was stripped of its atmosphere by the black hole's powerful gravity.

Astronomers believe stripped stars circle the central black hole of our Milky Way galaxy.

To find this event, Gezari's team monitored hundreds of thousands of galaxies, looking for a bright flare in ultraviolet light from the nucleus of a galaxy with a previously dormant black hole. They found one in June 2010.

They then monitored the flare as it reached peak brightness a month later and slowly faded during the next 12 months.

"The longer the event lasted, the more excited we got, because we realized this is either a very unusual supernova or an entirely different type of event, such as a star being ripped apart by a black hole," says Armin Rest of the Space Telescope Science Institute in Baltimore.

By measuring the increase in brightness, the astronomers calculated the black hole's mass to be several million suns - about the same as our Milky Way's black hole.